Wireless communications services within a cellular network are provided through individual geographic areas or “cells.” Historically, a cell site has generally included a cellular tower, having RF antennas that communicate with a plurality of remote devices, such as cellular phones and paging devices, and a base terminal station (BTS). A BTS typically includes one or more radio frequency (RF) power amplifiers coupled to the RF antennas for transmitting wireless communication signals to the remote devices. Cellular networks may provide services using digital modulation schemes. Such modulation schemes may include time division multiple access (TDMA), code division multiple access (CDMA), and Global System for Mobile communications (GSM), as well as others.
In a theoretical or an ideal RF power amplifier, the output power of the amplifier is equal to the input power of the amplifier multiplied by a constant (K), i.e., the amplification or gain factor, and does not vary with input power level. Moreover, the phase of the output signal is the same as the phase of the input signal. Practically, both the output power and phase vary as a function of the input signal.
Generally, an RF power amplifier has three operating regions. The first region, or linear region, includes operation where input signal power levels are relatively small and K remains constant. In the linear region, the response of an amplifier closely approximates that of an ideal amplifier. The second and third regions are referred to as non-linear regions. The second region or compression region begins where input power levels have increased to the point that K begins to reduce or roll-off with further increases in input power. The third region or saturation region is where the output power of the amplifier fails to increase with an increase in input power.
Transistor devices used in RF power amplifiers are coupled to a direct current (DC) power supply, and are biased, in part, based on the voltage of the DC power supply. The selection of the bias point or quiescent operating point of the transistor device also determines the efficiency and the distribution of intermodulation distortion products generated by the device, and thereby determines the spectral purity of signals amplified by the RF power amplifier.
Transistors particularly suited to operation in the 500 Megahertz (MHz) to 3.5 Gigahertz (GHz) range and that are typically used in RF power amplifiers in BTS are often lateral diffusion metal oxide semiconductor (LDMOS) devices. LDMOS devices offer excellent thermal characteristics due to advanced flange materials. Other suitable transistors may be GaAs, SiGe, SiC, and GaN devices.
In light of the foregoing, RF power amplifiers may be characterized and compared using their efficiency and spectral purity. RF power amplifiers may also be selected for a particular application based on their efficiency and spectral purity. Moreover, RF power amplifier suppliers often differentiate themselves based on efficiency and spectral purity.
Unfortunately, efficiency and spectral purity are typically mutually exclusive. For example, in order improve spectral purity the output power of the RF power amplifier must be reduced, causing an increase in the thermal dissipation of transistor devices in the RF power amplifier while the efficiency suffers. Conversely, in order to increase the output power and improve efficiency, the spectral purity of the RF amplifier often suffers.
Increased demands on the RF spectrum caused by continued increases in the number of wireless communications users require more spectrally efficient modulation schemes, such as the aforementioned digital modulation schemes, and more efficient RF power amplifiers for use therewith. Moreover, wireless devices used with cellular communications systems require high power efficiency for purposes of extending battery life. Thus, RF power amplifiers used in such systems are often operated close to or in a non-linear region to maximize efficiency.
Unfortunately, operation near a non-linear region may result in non-linear operation when digital modulation schemes having a fluctuating envelope are used. For example, operation in compression may result in distortion and spectral spreading into adjacent channels. The Federal Communications Commission (FCC), like other governing bodies, limits emissions in adjacent channels. Band broadening and/or an increase in the bit error rate may also result. Governing bodies typically limit out of band emissions as well. In some instances, the number of calls a system is capable of supporting may be reduced.
Techniques developed to reduce non-linearities in amplifiers may be categorized as either feed-forward, feedback or predistortion, each having their respective advantages and disadvantages. The feed-forward technique attenuates a portion of an RF power amplifier output signal so that it is the same level as the input signal. The difference between this distorted output signal and the input signal is used to generate an error signal. The error signal is then amplified and subtracted from the RF power amplifier output, improving the linearity of the RF power amplifier. Generally, feed-forward techniques are capable of handling multi-carrier signals, but do so at the expense of efficiency.
The feedback technique uses synchronously demodulated output signals as the feedback information, forming a feedback loop. These signals are subtracted from the input signals, generating loop error signals. If the feedback loop gain is sufficient, the loop error signals continuously correct any non-linearity in the RF power amplifier response. Generally, feedback techniques used with RF power amplifiers provide a reduction in out of band emissions, while being easily implemented. However, stability requirements limit bandwidth due to a dependence on loop delay. Thus, feedback techniques are of limited utility when used with certain modulation schemes.
The predistortion technique provides an appropriately distorted signal to the RF power amplifier, so that the RF power amplifier output is a scaled replica of the input signal. One type of predistorter uses a fixed signal predistortion circuit prior to amplification. A fixed type predistorter is of limited utility when used with digital modulation scheme having a fluctuating envelope, and does not account for changes, or drifts, in RF power amplifiers used therewith.
Another type of predistorter is an adaptive predistorter. In one adaptive predistorter the amplitude modulation to amplitude modulation (AM-AM) and the amplitude modulation to phase modulation (AM-PM) characteristics of an RF power amplifier are estimated, using cubic spline interpolation, from a look-up table of distortion values generated using synchronous demodulation from the RF power amplifier output. The estimated values are then used to predistort the input signal to the RF power amplifier. The performance of an adaptive predistorter is typically comparable with that of negative feedback and feed-forward techniques without being limited in the modulation scheme used or suffering from drift.
Generally, such an adaptive predistorter operates as follows. First, a digital signal or a baseband signal is encoded into in-phase (I) and quadrature-phase (Q) components. The I/Q components then pass through a pulse-shaping filter to ensure free-symbol-interference (FSI). The I/Q signals are then applied to a squaring circuit that produces a scalar value (Vm)2 indicative of the power of the baseband input signal. The scalar value (Vm)2 is then used as a pointer to a look-up table that contains predistortion values for the I/Q components. The predistortion values are then multiplied with the I/Q components, generating predistorted signals Id and Qd, respectively. The predistorted signals Id and Qd are then converted to analog signals and applied to a quadrature modulator. The quadrature modulator, driven by an oscillator, generates a modulated RF signal that is applied to the RF power amplifier.
A portion of the RF power amplifier output is applied to a quadrature demodulator, driven by the same oscillator, to produce I/Q baseband signals. The I/Q baseband signals are converted into digital signals (I′/Q′). I′/Q′ are then compared to I/Q, respectively, to estimate the AM-AM and AM-PM characteristics of the RF power amplifier. Since there is a delay in time between when the predistorted signals Id/Qd are applied to the RF power amplifier and the time that digital signals I′/Q′ are developed, the input signals I/Q must be delayed by that same amount of time before making the comparison. Thus, such a predistorter, in comparing I/Q signals, may be said to be “correlated” and “adaptive” in that the values in the look-up table change with time.
Such correlated adaptive predistorters may use cubic spline interpolation in estimating the AM-AM and AM-PM characteristics for values of (Vm)2, using values stored in the look-up table. Accuracy equivalent to that afforded by cubic spline interpolation requires a high order polynomial for a single polynomial fit. Although the use or application of cubic spline interpolation avoids the need for higher order polynomials in linearizing the response of an RF power amplifier, such correlated adaptive predistorters are still complex and costly by virtue of the delay and demodulation circuits used therein.
Although predistortion is one technique that may be used to improve the spectral purity of a RF power amplifier without reducing the efficiency of the amplifier, one problem with predistortion is the improvement to spectral purity is quickly reduced, or thwarted, if clipping of the waveform occurs in the amplifier. For example, the waveforms typically associated with communications system using the IS-95, CDMA2000, and Universal Mobile Telecommunications System (UMTS) communications standards all may experience peak power levels that may be anywhere from 6 to 12 decibels (dB) greater than average power levels. As a result, a high probability of clipping RF power amplifier used in such systems exists.
There is a need to maintain spectral purity in a RF power amplifier in the presence of peak power clipping without sacrificing efficiency.